Orthogonal frequency division multiplexing (OFDM) is the downlink modulation format used in the 3GPP Long term evolution (LTE) standard. An OFDM symbol with a duration of Tu consists of many sub-carriers, each having a sub-carrier spacing of Δf=1/Tu. The number of sub-carriers Nc and the sub-carrier spacing determines the overall transmission bandwidth of the OFDM signal which can be approximated as BW≈Nc×Δf. In order to maintain sub-carrier orthogonality in time dispersive channels, each OFDM symbol is prefixed with a cyclic extension of duration Tcp. The total duration of an OFDM symbol then becomes Ttot=Tu+Tcp. In principle, the cyclic prefix (CP) length should cover the maximum length of the expected time dispersion of the channel. However, the CP represents an overhead cost and hence it is also beneficial to keep it as short as possible.
In LTE, two CP durations are defined. A so-called “normal” (shorter) CP is defined to be suitable for small and medium cell environments. A so-called “extended” (longer) CP is also defined to be used in environments with extreme time dispersion, especially in the cases of single frequency network (SFN) operation or Coordinated Multi-Point transmission (CoMP) operation. A longer CP provides more robustness towards channel delay spread at the cost of more overhead. The LTE frame structure is illustrated in FIG. 1a. The normal and extended CPs defined within LTE are also illustrated in FIG. 1a. Further, a principle for deriving a CP is illustrated in FIG. 1b. FIG. 2 shows a number of sub-carriers, and illustrates the sub-carrier spacing discussed herein.
Regardless of the CP duration used, the basic time structure in LTE is unchanged. A 10 ms radio frame consists of 10 sub-frames regardless of the CP duration, as illustrated in FIG. 1a. The different CP durations are seen when examining how many OFDM symbols that fit into a 1 ms sub-frame. A normal CP sub-frame contains 14 symbols (7 per slot) while an extended CP sub-frame contains 12 symbols (6 per slot). The CP duration of an LTE carrier is blindly decoded by the UE as part of the cell search procedure.
In the standardization forum 3GPP LTE Rel-12, discussions are ongoing at the moment to define a new carrier type (NCT). Even though it is not yet defined what the NCT will end up becoming, one likely characteristic is that it will contain mandatory transmissions only in sub-frames 0 and 5. In all other sub-frames, a base station that does not transmit any user data will not transmit anything at all, i.e. not even reference signals. This reduces the interference from reference signals, also known as pilot pollution and it enables the base station to save energy by means of discontinuous transmission (DTX).
In OFDM the CP duration should reflect the maximum expected delay spread of a carrier. All users that attach to a cell will determine the CP duration of the cell blindly.
A large CP may be required in case the channel has a large maximum delay spread, in order to prevent that system performance will be degraded by inter-symbol and inter-carrier interference.
It is realized by the inventors that with current state-of-the-art solutions, an unnecessarily long CP is used in many situations, in order to support UEs associated with large delay spread. However, for many UEs, which do not experience a large delay spread, this long CP degrades the performance.